Cron Help (To use a cron?)

So I want to have a delayed alerts system where a user is reminded to come back and look over their thread after X configurable time.

I was thinking the most efficient way to do it is this:

During thread creation if the thread is in the right forum, set a value in the dataregistry saying the alert needs to be set, with the thread ID and the time the alert is due (like update an array or something)

A cron runs every 4 hours and checks this dataregistry value to see if anything needs to be done. If it does, it sends the alerts out and removes the value from the DR.